1. Field of the Invention
The present invention relates to a thyristor with insulated gates.
2. Description of the Related Art
A thyristor of the voltage control type using an insulted gate electrode (MOS gate) is suitable for gate driving in a power device with a high breakdown voltage and a large current, since gate driving can be performed by a small current as compared with the thyristor of the current driving type.
FIG. 64 shows the structure of a turn-off insulated gate in the conventional thyristor of the insulated gate type. A p-type base layer 2 is formed on one surface of an n-type base layer 1 having high resistance. An n-type emitter layer 3 is formed in the p-type base layer 2. A p-type emitter layer 4 is formed on the other surface of the n-type base layer 1. A cathode electrode 5 is formed on the n-type emitter layer 3 and an anode electrode 6 is formed on the p-type emitter layer 4.
An n-type drain layer 7 is formed at the position, which is away from the n-type emitter layer 3 at a predetermined distance. A gate electrode 10 is formed on the p-type base layer 2 via a gate insulating film 9, and between the n-type drain layer 7 and the n-type cathode layer 3. The gate electrode 10 is used for turn-off and comprises an n channel MOSFET in which the n-type emitter layer is used as a source. A drain electrode 8 is formed in contact with the p-type base layer 2, and the p-type base layer 2 and the n-type drain layer 7 are short-circuited by the drain electrode 8.
A gate electrode for turn-on (not shown) is formed at a peripheral portion of the p-type base layer 2, which is selectively diffused, and comprises a MOS structure similar to the gate electrode for turn-off.
According to the above-structured thyristor of the insulated gate type, a positive voltage with respect to the cathode is applied to the insulated gate electrode 10 at the time of turn-off. Thereby, an n-channel is formed under the gate electrode 10. Then, a part of hole current, which has directly flowed into the n-type emitter layer 3 from the p-type base layer 2, changes its passages and flows into the drain electrode 8 as shown by a broken line, and passes through the n-type drain layer 7 and the portion under the gate electrode 10. Thus, the hole current is bypassed to the cathode electrode 5 from the n-type emitter layer 3. By the bypass of the hole current, injection of electrons to the p-type base layer 2 from the n-type emitter layer 3 is stopped, and the device is turned off.
In the conventional thyristor with the insulated gate, there is a problem in that sufficient turn-off capability cannot be obtained. This is due to resistance of a hole current bypass passage shown in FIG. 64. As resistance of the hole current bypass passage, there are mainly horizontal resistance of the p-type base layer 2 and on-resistance of the channel under the insulated gate electrode 10. If voltage drop, which is determined by these resistance and the bypass current, becomes higher than a built-in voltage between the n-type emitter layer 3 and the p-type base layer 2, injection of electrons from the n-type emitter layer 3 is not stopped. Due to this, if the main current increases, the device cannot be turned off.
As described above, in a conventional thyristor with insulated gates, a large turn-off current cannot be obtained.